An out-of-control wildfire near some of Colorado's most visited tourist sites has expanded and kept some 6,000 people from their homes, threatening a gated community nestled in the foothills near the famous Garden of the Gods.

The fire sent a mushroom cloud of smoke nearly 20,000 feet (6,100 metres) into the air over Colorado Springs, shadowing Pikes Peak, whose vistas helped inspire the patriotic tune "America the Beautiful." Closer to the blaze, trees were visibly twisting from the heat.

A famous cog railway that transports tourists from around the world up the picturesque mountainside said it would be closed on Tuesday for a third straight day. The highway that leads up to Pikes Peak has been closed since the fire began nipping at its base over the weekend.

The closures on Pikes Peak, billed the world's second-most visited mountain after Japan's Mount Fuji, have drawn attention to the fire's negative impact on the tourism industry just at the start of the peak summer season.

Fire crews called in massive C-130 military planes on Monday, which swooped low through plumes of black and white smoke to dump fire retardant on the flames.

Firefighters on the ground concentrated on the gated upscale subdivision of Cedar Heights, overlooking the Garden of the Gods whose towering red rock formations jut up from the ground, trying to protect residential houses from nearby flames.

Raging about 130 km south of Denver, the Waldo Canyon fire had initially prompted the evacuation of 11,000 people at the weekend although residents of the town of Manitou Springs were allowed home on Sunday night.

One of those residents was retired nurse Carol Yeager, 76, who left her stucco home on the orders of police late on Saturday, grabbing a handful of personal effects and her four cats.

"The flames were shooting skyward," she said. "It's cleared up a lot, but police told us if the wind shifts, it's touch and go, and they said to be ready to evacuate again."

FIRE NEAR AIR FORCE ACADEMY

The fire was also within 10 to 16 km of the Air Force Academy in Colorado Springs as winds fanned the flames in its direction, and fire authorities listed the academy as being threatened by the blaze along with utilities and watershed.

A recreation area belonging to the Academy was ordered evacuated due to its proximity to the fire, and all trails leading west of the school were closed, the base said.

The blaze would still have to traverse rough terrain, burning down through steep canyons and up mountain ridges, before it could reach the Air Force Academy itself, Academy spokesman Meade Warthen said.

"We don't have any reason at this particular point to think we're going to be inundated, but we're standing by," he said. "There are contingency plans in place. If we need to implement them, we will."

The Department of Homeland Security said it was providing the Federal Emergency Management Agency financial assistance to help battle the Waldo Canyon blaze and other fires, noting the threat to 250 homes in the area.

The blaze ignited as firefighting resources were stretched by the monster High Park blaze northwest of Fort Collins, a university city north of Denver along the state's tinder-dry Front Range.

"We're going to be continuing to have to deal with these fires for weeks to come," US Forest Service Chief Tom Tidwell said. "We anticipate it's going to be a long fire season."

The High Park Fire - the second-largest blaze on record in the state and its most destructive - has consumed 83,205 acres (33,670 hectares) in steep canyons since it was sparked by lightning two weeks ago. It is blamed for the death of a 62-year-old grandmother in her mountain cabin and has destroyed 248 homes.

An estimated 4,300 people remain evacuated from their homes as that fire burns through grass, brush and Ponderosa pine.

In southwestern Colorado, the Weber Fire grew to 8,300 acres (3,360 ha) overnight but firefighters held it about one mile southeast of the small town of Mancos, east of Mesa Verde National Park. Roughly 50 homes were evacuated, officials said.

Meanwhile, in neighboring Utah, a fast-growing 39,000-acre (15,780-ha) wildfire continued to rage largely out of control on Monday after burning an estimated 30 homes and killing 75 sheep between the rural communities of Fountain Green and Indianola.

Governor Gary Herbert, who toured the fire by helicopter on Monday, estimated the property losses so far at US$7 million. No injuries have been reported, but Herbert said fire officials did use a helicopter to rescue some shepherds from the fire's path.

More than 500 structures have been threatened by the Wood Hollow fire, forcing up to 1,500 people from homes.

"The big worry now is the weather. Everything that can be done is being done," Herbert told a televised news conference.